The World's Greatest Servant
by tuntuntunaa
Summary: Instead of a certain red-clad Counter Guardian, Rin summoned a sadistic baby with fedora. Enough said.


**Omg yes I know I've said that I won't guarantee the next chaps of FoS and WaC BUT I COULDN'T HELP POSTING THIS OR ELSE I'LL REGRET IT THE MOMENT SCHOOL STARTS TOMOROW.**

 **Why don't I continue FoS or WaC, you may ask? Well, for FoS, I do have written chapter 8 and now waiting for my beta Switchbladesunsets to edit it, so it'll definitely get updated soon. As for WaC, I got stuck in the next chapter and it's just so frustrating not to be able to think of a good, funny scene for Rin's first meeting with the twin Emiyas. Sorry for that!**

 **So yeah, my brain decided to betray me and instead allowing this stupid idea to pop into existence. It's just a plot bunny, but I feel really bad for not sharing it to the world.**

 **Disclaimer: I don't own KHR nor FSN. I do wonder how Amano Akira could make shameless humour in the beginnings of the KHR manga. Poor Tsuna, really.**

* * *

 **The World's Greatest Servant**

Katekyō Hitman Reborn! x Fate/Stay Night: Unlimited Blade Works

Unbetaed

* * *

There was a boom, and Tohsaka Rin shrieked in surprise as she landed on her butt.

The basement where she had performed the Summoning Ritual was now in a mess. Books were scattered on the ground, papers flew around the room and a big cloud of dust rose, making her cough violently. Even then, her chest swelled with pride as she excitedly exclaimed what a flawless ritual it was.

"I knew it! I've managed to draw the most powerful card - huh?"

Rin looked around her, searching for her summoned Servant, but no one else was in the room.

Suddenly, there was a loud crash from above. She ran up the stairs, cursing all the way, because surely that must be her Servant. How did he/she end up there, Rin had no idea.

On her way to the living room, she caught sight of a grandfather clock at the end of the corridor. Its little hand was pointing at number 2. Rin sighed in relief, because she thought she had forgotten that the clocks in her house were all one hour fast. At least, she got the time right, because her mana level was just at its peak at 2 A.M.

Finally, she arrived at the room where she believed her Servant had crashed. But the door was jammed, making her groaned in frustration as she tried to get through it by force. By the time the damn door opened (more like fell of its hinges because apparently she was heav- She meant _strong_ enough to slam it open), she had completely lost her composure.

Maybe that was why Rin failed to comprehend a squeaky voice saying, "Ciaossu," or see a tiny foot flying toward her face.

"Ack!"

Rin fell on her butt for the second time in five minutes.

Rubbing her nose in pain, she glared at her assailant as her mouth prepared to unleash expletives. But what she saw left her mouth hanging dumbly instead.

It was a...a baby.

He was in a black suit, with a black fedora with an orange stripe covering most of his spiky black hair. A green chameleon was lounging on the brim of his hat, and an orange...pacifier? rested on his tiny chest. The infant had the largest onyx-coloured doe eyes she had ever seen.

And, oh, don't forget the weird side-burns, because they must have disregarded Gaia's laws -

 _ **Bang!**_

Her left ear rang, but Rin didn't have to turn to see a bullet hole on the wall because the green gun - which appeared on the baby's hand out of nowhere - was now smoking for obvious reason.

"Did you just insult my side burns?" the baby asked innocently.

Rin spluttered, "N-no, I did not!"

The baby hummed. He shot at her once more, forcing Rin to scramble away from his firing line.

"Why did you shoot me again?!"

"You lied."

 _He can read my mind?!_

"I can. I just did."

"How?!"

"You ask too much question."

He shot at her again.

* * *

After painfully long minutes of jumping around like a monkey in an attempt to dodge the bullets, Rin was finally given a reprieve.

She plopped on the ground, panting in exertion. Rin eyed the baby warily, because even after the hell he had put her through, none of the bullet had actually hit her.

"You want me to shoot you for real?" the baby cocked his green gun menacingly.

"N-no," she stuttered. He could legit read her mind, she thought.

Rin quickly changed the topic of their conversation, "That gun...I've never heard of it, but I guess you're an Archer since you possess a main projectile weapon, even if it's not a bow but a modern firearm."

The baby didn't refute her statement, so she assumed it was correct.

She continued hesitantly, "You're my Servant, right?"

He smirked, "Who knows?"

Her eyes twitched. She took a full minute of silence to close her eyes, regulate her breathing and regain her composure.

"I'm simply asking if you really are my Servant and no one else's," she said without opening her eyes, because Rin didn't want to see the disturbing smirk that wasn't supposed to be on a baby's face, for god's sake. "Master-Servant relationships should be clearly laid out, right off the bat."

No answer.

Taking his silence as an agreement, she carried on speaking, "You don't look like it, but I acknowledge you as a Heroic Spirit with unimaginable power. Well, initially I wanted a Saber-class Servant, though after seeing your...skills with your gun, I guess it can't be helped. Despite that, bear in mind that I, as your Master, will also actively participate in the War. I won't sit back and only watch Servants fighting - because I'll win this War with my own hands..."

There was only silence. Rin was confused. She slowly opened her eyes, only to see the baby sleeping with his eyes open, the large bubble coming out of his nose the only indication that he was sleeping.

Rin pulled her hair out in frustration.

* * *

Thankfully (or not), the baby woke up a few minutes later.

Instantly, his green chameleon turned into a green gun (it turned back into the lizard when he was sleeping. Was it some sort of a Noble Phantasm?). He fired at her, making her yelped in fright.

"Why?!"

"You talked too much until I fell asleep."

 _ **Bang!**_

"Now what?!"

"I just feel like it."

Rin tried to reign in her patience. For now, it worked.

At least, when the baby was sleeping, she managed to calm down and think through her current situation. The brief silence also gave her the chance to find out her connection with the baby in front of her, so if nothing else, she could confirm that he was indeed her Servant.

"By the way," she started. "Which Heroic Spirit are you?"

"Hm? I'm Reborn."

She drew a blank.

"Is that your real name?"

"Of course, Baka-Rin."

"Wait, how did you know my name? And I'm not stupid!"

"Because I'm the world's greatest hitman."

She pulled her hair out and groaned, "That doesn't explain anything!"

After a few moments of processing his reasoning, she retracted her statement, "Actually, that explains some things, like why you're wearing a suit or why your weapon is a gun or why you have a good...information gathering skill."

 _Though how did he do that while he was with me the entire time beats me. Maybe he looked for my name as I was making my way upstairs._

Reborn hummed, "At least, you're not as stupid as my student."

...She wasn't sure if that comparison was a compliment or an insult.

"Anyway, you're not a real baby aren't you?"

 _ **Bang!**_

"Never mind! I asked too much question. I know the answer already," she deadpanned.

"Glad you speak my language now," Reborn said smugly.

* * *

Reborn pulled down the brim of his hat, turning serious all of sudden.

"Why do you want to win the Holy Grail War?" he asked.

Rin gave him the automatic answer, "For generations, winning the Grail has been the Tohsaka dream."

Reborn shot at her. She ducked, but for the first time since their exchange, she was harmed. Rin winced in pain as her cheek was grazed.

"Your true wish," he demanded, clearly dissatisfied with her answer. "That's not one. I refuse to support a Master without a clear goal, because the Grail can grant any wish - no matter how outrageous it is."

She frowned. Rin was a bit scared, because even after playing around with her, the baby was still a Heroic Spirit: a being blessed with inhuman capabilities. But she steeled herself and replied in determination.

"I don't know."

For once, Reborn was surprised, "You don't know?"

Rin nodded.

"I don't know," she reiterated. "True, the Grail has the power to even allow someone conquer the entire world or anything like that, but to me it sounds like a pain in the ass."

"Then, why are you even participating in this War?"

Rin answered confidently, "Because there's a battle to be fought, Archer. I'm fighting to win, that's all."

There was silence, then -

 _ **Bang!**_

"Why - "

"I'm not satisfied with your will," Reborn interjected her squawk. "You'll die before you could win. You might as well hide in the cellar and wait for me to win it."

She bristled at his statement. But before she could say anything, he continued on speaking.

"That's why, I'm going to to- I mean _tutor_ you to be the best Master there is, and then win the War."

...Rin shivered.

* * *

 _ **Bang!**_

"Why suddenly?!"

"Now, your first lesson is to clean this room."

"Wait, wait, wait. Isn't that a Servant's job?!"

 _ **Bang!**_

"A good Master must keep her house clean and spotless."

"But you're the one who made the mess!"

 _ **Bang!**_

"I had no choice but to appear here since you stood on the Summoning Circle. You were in the way, Baka-Rin."

 _ **Bang!**_

"And this is for talking back to me."

"ENOUGH!" Rin snapped at her Servant. She hold up her left hand, and shrieked, "I COMMAND YOU TO STOP BULLYING ME WITH YOUR DAMN GUN!"

There was a shockwave, and a silence followed.

Then…

"That's the dumbest command I've ever known of," Reborn remarked.

 _ **Whack!**_

"Ack! What - Why - Where did the green mallet come from?!"

* * *

The next morning, Reborn woke her up by kicking her out of the bed. Literally.

"Ouch! Archer!" she whined.

"You're late for school," Reborn said.

Rin had expected him to violently make her go to the school in record time - because the green mallet was already on his hand, duh.

Instead, he said, "You're mana-exhausted."

Rin blinked at that.

"Ah, you're right," she hummed tiredly. "I guess since Masters are the ones sustaining their own Servants with mana, your summoning makes me tired."

Reborn nodded.

"Since you're tired, I'll let you skip school for today. We'll be going around the town in the afternoon."

She sighed in gratefulness.

"...Wait, then why did you wake me up?"

"I just felt like it."

Rin facepalmed.

* * *

They had been wandering around the town for about an hour. For some reason, Reborn refused to astralise, and so he perched on Rin's head without her permission.

Occasionally, Reborn stopped her and hopped down to disappear somewhere, before coming back around fifteen minutes later.

"...What are you doing?"

"Making secret bases. Better be safe than sorry."

'So, that's what he's been doing?! Digging holes on the ground or dismantling walls to hide stuffs like a dog hiding its bones?!'

"But this is someone else's house!" Rin exclaimed. "How did we even get in here, anyway?!"

Reborn ignored her, of course.

She sighed, turning to see whose house it was from the nameplate.

'Shit, this is Emiya-kun's house!'

She had told no one, but Rin had been attracted to this one boy named Emiya Shirou after witnessing his stubborn attempts to do a high jump, even if it was proven impossible for him. It was years ago, and she even knew of her former-sister's crush at him, but she couldn't help admiring his resolve.

He was also kind-hearted, and -

Rin shivered as she felt eyes on her. She looked down, only to see her Servant smirking at her and gave her a knowing look.

"So, you do have a wish…"

"W-what are you talking about?" she feigned ignorance. _Damn his mind-reading ability!_

Reborn shrugged. He walked out of the house and away from it, leaving Rin to scramble after him.

"I'll know soon enough," he said mischievously.

Rin dreaded that moment.

* * *

They were on a rooftop of a tall building in the evening when Rin spotted Emiya Shirou on the crossing bridge right below.

She quickly casted her eyes away, but it was too late.

Reborn must have noticed the red-haired boy, because he covered his mouth and giggled disturbingly.

Maybe she should skip school tomorrow too.

* * *

She didn't get to skip school on the next day because, oh, hello green mallet.

* * *

For once, Reborn wasn't in her vicinity.

Rin slumped on her seat in relief, happy that she was finally free of that sadist.

Her mind went back to the third-rate boundary field surrounding her school. She frowned, thinking of possible suspects that dared to harm the people who were in her territory. The students and teachers were automatically under her protection, so she had to quickly dismantle the boundary field before it could negatively affect them.

Her intense pondering made her fail to notice her homeroom teacher making an announcement that their Mathematics teacher was now on a temporary leave, and there would be someone else to replace him.

Her thoughts screeched to a halt when her eyes caught sight of a certain spiky haired baby wearing a typical teaching attire entering her class.

"Ciaossu," said the baby-teacher that _must_ be her Servant. "I am Mr. Reboyama, and I will be your substitute Mathematics teacher from now on."

Oh, hell.

* * *

Funny thing was, nobody had said anything about a baby teaching the class mathematics. Students and teachers alike took it in stride as if it was normal.

Except, it _wasn't_ , dammit! Why nobody had noticed anything?! Why was it only her who could notice this...this ridiculousness?!

Suddenly, Mr. Reboyama smacked her with his dear green mallet in the middle of the class.

"Pay attention, Tohsaka-san," he commanded.

Rin gaped at him. Noticing her classmates whispering at each other and pointing at her for her unusual demeanor, she quickly regained her composure and gritted out a, "Yes, sir".

Fifteen minutes of dealing with the resulting headache later (all because of the damn baby), Rin almost smacked her own face as she finally realised her own stupidity.

Reborn was a Servant; of course he had Skills that reflected the abilities he had in life.

Though, she wasn't sure what this particular skill of his is called. Cosplaying, Rank A?

A snort escaped her, prompting Mr. Reboyama to whack her with the green mallet again.

* * *

"Are you okay, Tohsaka?"

"Huh?" Rin whirled around to see Emiya Shirou looking at her in concern.

She blinked, then turned back to see Reborn already gone. She had been talking to him about the boundary field in the school and her plan to dismantle it tonight when Shirou called her out.

"Ah, I mean," Shirou began, rubbing his head shyly. "You got hit by Mr. Reboyama's mallet multiple times. That might get you a concussion."

Rin's eyes widened.

"You can see the mallet?" she asked tentatively.

"Yeah, of course," Shirou replied. "Though, I don't know why the others can't see it." He muttered something else under his breath, suspiciously sounding like 'and why others can't see that he's _a baby_ ', but she may have heard it wrong.

"Sorry, what?" Rin inquired, leaning forward to better hear what he said.

Shirou blushed because of their close proximity, though sadly Rin didn't see it. She was facing sideways to give her ear to him.

"N-no," he stuttered. "Nothing."

"I...see," she said slowly. "Well, to answer your question: I'm fine, Emiya-kun, so don't worry."

As the boy walked away, she began to grow suspicious of the redhead. Why could he see the mallet, but others couldn't?

Rin shook her head. Shirou is a civilian, not a magus. It doesn't matter.

"It matters a lot, Baka-Rin."

Rin yelped at the sudden squeaky voice that was her sadistic Servant's. She whirled around to find Reborn cocking his gun up.

"Lesson time," Reborn began abruptly. "Number one: Do not dismiss any suspicion that your intuition has told you. You may initially dismiss Emiya Shirou as ordinary, but it takes someone who is at least adept at Structural Analysis to be able to see through my A-ranked Cosplay ability."

Rin would have started at the fact that Emiya Shirou was a magus if not for Reborn's confession of his ability.

"Pfft! I was right - "

"Lesson number _two_ ," Reborn continued, his eyes glinting in promised pain for her daring to laugh at him, "A good Master should be straightforward and resolved, especially when it is a matter of personal feelings for others."

Rin spluttered, her face red from the implication, "W-what! No! I - "

"You shouldn't have left his feelings hanging."

"He wasn't even confessing at me!" Rin flailed around, trying to stop this madness Reborn was sprouting. "B-besides, my sis - I mean, S-sakura already has a c-c-crush on him!"

Reborn seemed to have enough of her denial, because he aimed his gun at her head.

"It's not good to bottle up your feelings," he declared. "So, show me your dying will and _die_."

Rin screamed in terror. She raised her arms to cover her head, waiting for the gunshot...that had never come.

"Tch," Reborn pouted, his gun still pointing at her but it was shaking. "Stupid Command, ruining my fun."

He stole a glance at the hall where Shirou had gone to ten minutes ago. His lips widened into an evil smile as a glint appeared in his beady eyes.

"I guess I still have a backup target."

Rin could do nothing but swallowed hard, dreading for what was to come.

* * *

It was in the evening when things started to get serious.

Rolling on the rooftop floor, Rin managed to dodge the red spear's tip by a hair's breadth. A millisecond later, she had to spin around to avoid another attack. She wasn't given time for reprieve, as a moment of hesitation could mean instant death in the hands of the blue clad Servant.

A small part of her mind idly commented that her current situation wasn't unfamiliar at all, because a certain baby had made sure that she could at least dodge high speed attacks against her.

At least, Reborn would make sure Rin wouldn't die from his sadistic training (...right?). On the other hand, this Servant was an enemy Servant, whose sole purpose of existence was to kill other Masters like her.

Matters became worse when she realised that the rooftop was too space-confined, fenced in on four sides, making it difficult for her to gain a distance from her enemy.

She had to take drastic measure, then. And a leap of faith for her own Servant.

"Es ist groß. Es ist klein," chanted Rin. Glowing runes appeared on her legs, indicating reinforcement in a preparation for a daring jump. "Vox Gott es Atlas!"

She leapt over the fence, and for a brief moment Rin felt weightless before the gravity pulled her down.

"Archer," she hollered, tightly scrunching her eyes shut lest she lose her bravado. "I'll leave the landing to you!"

Rin didn't expect to hear a gunshot, though, nor did she expect to receive a headshot.

* * *

 _When dying, Rin regretted._

' _What a waste...If I had the will of dying, I probably could've been less dependent on my sadistic Servant and believe in my own power, instead of stupidly dying from falling from an extreme height…'_

' _I want to survive…'_

* * *

Rin's falling, limp body suddenly bulged and, lo and behold, emerged _another_ Rin with eyes ablaze, forehead in fire and in nothing but her undergarments.

"REBORN!" she yelled on top of her lungs like...like her _life_ was on the line. "Land safely on the ground and survive WITH MY DYING WILL!"

Alas, it was no use screaming. Rin had jumped too far from the school building, and now her flailing limbs couldn't even reach the wall -

 _ **Bang!**_

Another bullet hit her head out of nowhere, and in an instant Rin's scalp felt extremely itchy.

"It itches! My hair whorl…," (Rin had no hair whorl, yes - her hair was straight - but for the sake of this moment just...just accept that she had it for some reason) "ITCHES!"

A...coil spring made of, supposedly, Rin's _hair_ grew out of the center of her scalp, just in time to aid in her landing process by allowing her to bounce harmlessly on the ground and stop with her body intact.

There was a moment of silence, before another figure - distinctively blue - fell painfully on the unforgiving ground with severe nosebleed.

Reborn soon followed in a much more graceful manner, landing beside a disoriented - and soon, mortified beyond measure - Rin with a satisfied smirk.

All was well.

* * *

"Why am I naked?! And how come I didn't die from the headshots?"

Reborn smiled at her, before showing her a bullet.

"This bullet is a Dying Will bullet, one of my Noble Phantasms," he began. "A person who is shot with this bullet will resurrect with dying will, which is based on what you were regretting the moment you die. Your body would be in a state wherein all safety switches are off, which means in exchange for risking your life by breaking your limit, you can harness amazing strength."

"I...see...," Rin said slowly. "It's like unlocking my maximum potential abilities. That's...unheard of."

Reborn shrugged, before smacking Rin with his beloved mallet.

"Jumping over the fence...That was stupidity in a whole new level, Baka-Rin."

 _ **Smack!**_

"Ow! Yes, yes! Sorry, that was my fault!" Rin cried out in resignation. "I was imagining that you...you'll..."

"..."

Rin coughed, embarrassed, "Okay, yeah, you're a baby. There's no way you could hold me and land me safely or anything."

Reborn snorted, "You fool. You're lucky I have experience with this kind of situation."

"...Someone had done this stupid stunt before?" Rin sweatdropped, before shaking her head. "Never mind that; how did you manage to shoot me? I thought the Command was in place."

"Apparently, shooting you for the purpose of aiding your cause is not considered as 'bullying you with my damn gun'," he answered smugly. Rin paled considerably. "So, I guess I should put you in life-and-death situations more before, ah, _aiding_ you."

All was well, indeed.

* * *

"Ah, you're finally back, Lanc...Are you okay?"

"Uh, I-I'm fine."

"You got blood all over your nose, and your face is red. Was it the enemy Servant? Who did you meet?"

"N-nothing in particular. I just...I just tripped, that's all!"

"..."

"..."

"Rin has never changed. I've been her guardian since she was but a child."

"..."

"Was it the teddy bear panties? She's rather fond of them."

More blood.

* * *

 **It actually started from the scene where Rin jumped over the fence and left her landing to Archer, who would later carry her bridal style and bring her safely to the ground. I was watching this scene in UBW when, suddenly, I got a stray thought: Hey, didn't KHR also have this kind of scene, where Tsuna saved Takeshi when they fell over the fence in the early chapters?**

 **From there on, bloom this hilarious thought of combining the two scenes together, and thus was born this fic.**


End file.
